I have experimented with forming these cases a lot. Here is what I learned.
All dimensions in inches.

To do the intitial set up of your dies you can use once fired .303. Do not mess with once fired cases for actual shooting.

Start your brass for shooting with new cases. Most of mine are made from Hornady Frontier cases.
Due to the current high cost of brass make sure you have a good setup so you do not waste new cases.

There are a number of variations you can use to form these cases but this is the best for me.

1. Expand the case mouths to remove all the dents.
2. Use a Pacific 7.35 Carcano sizer to push the shoulder back. (die is .315" ID)
This sizer moves the shoulder back and supports it with out a bulge like you will get with a .308
Win. die. The Pacific is large enough at the base that it does not size the base undersize.
3. Size the neck with a Pacific 6.5 Carcano trim die. (Neck is .300 ID) Does not size the base.
4. Size the entire case with with a 6.5X53R trim die. My die is made by RCBS. Set headspace to the
shoulder so you can feel a slight drag when the bolt closes.
5. You can size only the neck with a 6.5X55 die or use a FL sizer.

If you have a .250 Savage trim die you will find it very useful for forming these cases too.
The neck is about the right size (trim dies are normally .010 larger than FL sizers) and the
small diameter shoulder works too.

Many dies and operations yes
I normally pre-trim after OP 3 Then trim again after op 4. My chambers are much longer to the
case mouth than the 2.110 lenght given for trim to length. They measure 2.153. Since the .303
body is smaller in diameter than the 6.5X53R it blows out a bit when fire formed and gets a
little shorter. I trim to 2.150-2.152. When fire formed they shorten to 2.148-2.145. After the
brass is fired once I resize and trim to 2.145.

The Dutch chambers are .453" in diameter at the rear just ahead of the rim.
Check your new brass to make sure it is .452" or smaller.
I have measured PRVI and old lots of Norma brass that are larger than .453. So stay away from
oversize case heads.

If you use this process and reasonable care you can get 100% yield from your new cases.
I would recommend you anneal the necks after the first firing.